[dev.simd] cmd/compile: ensure the whole X15 register is zeroed

[dev.simd] cmd/compile: ensure the whole X15 register is zeroed

On AMD64, we reserve the X15 register as the zero register.
Currently we use an SSE instruction to zero it, and we only use
it in SSE contexts. When the machine supports AVX, the high bits
of the register is not necessarily zeroed.

Now that the compiler generates AVX code for SIMD, it would be
great to have a zero register in the AVX context. This CL zeroes
the whole X15 register if AVX is supported.

Change-Id: I4dc803362f2e007b1614b90de435fbb7814cebc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/698237
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: David Chase <drchase@google.com>